There are many electronic systems that include a host device that powers one or more remote electrical devices. For various reasons, the physical connection between the host and remote devices is often limited to a single wire pair, or power feed lines. In these types of devices, it is often useful or necessary to pass signals between the host and remote device in one or both directions. One method for achieving this without increasing the number of wires between the devices is to modulate a signal of interest with the power signal present on the power feed lines.
FIG. 1 is a schematic of a system 2 illustrating a typical prior art method for passing a signal from a remote device to its host device over a pair of power feed lines. As illustrated in FIG. 1, a host device 4 supplies power over a pair of power feed wires 5a and 5b. A remote device 3 is serially connected between the positive and negative power feeds 5a and 5b to complete the current loop. The power feeds 5a, 5b provide a potential VS1 across the remote device 3. A series impedance R6 is connected on the host 4 between the supply voltage VSUPPLY and the positive power feed line 5a. The negative (or ground) power feed line 5b is connected to the host circuit ground. In operation, the remote device 3 transmits the signal of interest by varying the current drawn in the power feed lines 5a, 5b. To recover the signal of interest, the host device 4 includes a differential amplifier 7, which measures the differential voltage 8 VOUT across the series impedance R 6. The current signal of interest is thus converted to a voltage signal and further processed by filter circuit 9.
The signal communication technique illustrated in FIG. 1 is problematic for several reasons. First, series impedance R 6 in the supply line 5a causes variation in the supply voltage of the remotely powered device. As with all circuits, the remote circuit 3 has limited power supply rejection. Supply voltage variations at the remote device 3 lead to degradation of the signal of interest (e.g., a measurement signal) or, in some cases, instability and oscillation.
Second, the value of the series impedance R 6 must be relatively low in order to minimize the voltage drop across the impedance R 6. Therefore, current-to-voltage gain adjustments are limited and overall measurement dynamic range is degraded.
Finally, a relatively complex differential amplifier 7 with high common mode rejection and matched components is required in order to sense the voltage across the series impedance R 6. Often this differential amplifier is AC-coupled which adds additional cost and complexity.
Accordingly, a need exists for a simpler, more robust technique for sensing remote current signals over a single pair of power feed wires.